1. Field of the Invention
The present invention relates to a robot controller and a robot controlling method.
2. Background Art
A multi-axis robot having a multi-link structure comprises a position feedback control system including an actuator (for example, an electric motor), which drives each axis, and an angular sensor (for example, a rotary encoder), which detects a rotation angle of each actuator. Many of position feedback control systems have used PID (proportional, integral and differential) control. Moreover, a semiclosed loop control, which controls the rotation angle of each link to indirectly control the angle of an actuator, has been used so far.
However, a torque transmission mechanism, which transmits torque of an actuator to a link, is not generally a rigid body, and is approximated by a reduction gear with a certain spring characteristic. Thereby, the multi-axis robot having a multi-link structure has caused a problem that the actuator and the link are vibrated each other, because the actuator and the link mechanically have a resonance characteristic.
In order to solve the above problem, there has been disclosed a state feedback control system for vibration control (refer to Japanese Patent Application Laid-open No. 63-251808), in which an angular sensor and an acceleration sensor are provided on a link for direct measurement of an angle and acceleration of the link.
However, in order to mount the angular sensor, a complex structure is required, for example, a projecting portion is required to be provided outside the axis of the link. Accordingly, the shape of the link is limited. Moreover, it is impossible to add the angular sensor to an existing multi-axis robot.
On the other hand, the acceleration sensor can be added on each link of the existing multi-axis robot (refer to “Introduction to Robot Control” published by CORONA PUBLISHING CO., LTD., pp. 238-239, 1989 (hereinafter, called Non-Patent Document 1)). However, it is general that the signal of the acceleration sensor has a small signal-to-noise (S/N) ratio, and is susceptible to gravity.
Furthermore, an advantage caused by feedback of the acceleration of the link is only change in the moment of inertia (refer to Institute of Electrical Engineers of Japan (IEEJ), Transactions on Industry Applications, Vol. 123, No. 2, pp. 149-155, 2003). Accordingly, in order to control the vibration of the link, feedback of the angular velocity and the angle of the link, in addition to that of the acceleration of the link, is indispensable. However, as the acceleration sensor, which can be obtained at a low price, can usually measure only a translational component, complex operations for coordinate transformation are required in order to obtain the angular acceleration of a link alone. For example, an operation, which has been disclosed as a formula A19b in the Non-Patent Document 1, is required, and, in this case, the angular velocity of a first link is separately required to be obtained in order to obtain the angular acceleration of a second link alone. Moreover, the angular acceleration is required to be integrated once to obtain the angular velocity of the links from the angular acceleration thereof, and the angular acceleration is required to be integrated twice to obtain the angle of the links from the angular acceleration thereof. As the angle, which is obtained after the angular acceleration is integrated twice, is susceptible to a drift, it is impossible to obtain an exact link angle.